Lincoln was the first president to be assassinated. If no one expects it then people are gonna lock up to gunfire and possibly not realize what it is. Look at the Shinzo Abe shooting. There were plenty of guards, just due to the situation in Japan they weren't at all prepared for an assassination or gunfire.
And he timed the shot to be at a point where everyone laughed during the play,meaning many people just had no idea what was going on.
So, what's your theory of what happened?